Herbaceous plants have a life cycle that includes seed germination, growth and differentiation, and flowering to prepare for next generation. In such processes, interactions with various hormones as the internal growth factors as well as external environmental conditions, including light, water, temperature and soil, have important effects on the growth and differentiation of plants. The most important factor in the growth of plants is considered to be the sugars produced by the photosynthesis process, which is the most fundamental phenomenon in green plants and is an important process for obtaining an energy source, required for the survival of plant, and the structure components of plant. So, these sugars would perform an important role in the growth and development of plants throughout the life cycle of plants.
Also, the sugar can act as signaling molecule like plant hormones to control the expression of not only genes related to photosynthesis, but also genes related to metabolic and developmental processes, which are important for the growth of plants, including respiration, the synthesis and decomposition of starch and sucrose, nitrogen metabolism, cell cycle regulation, and senescence. Thus, it can be seen that sugar-related reactions in plants are linked complicatedly in view of not only metabolism, but also functions as cellular constituents and regulatory factors.
Among such sugar-related genes, SPF1 (SWEET POTATO FACTOR1) was isolated from sweet potato for the first time, and is a DNA binding protein which recognizes an SP8 motif present in the promoters of sweet potato sporamin and β-amylase genes. SPF1 is known to act as a negative regulator which inhibits the transcription of a target gene by sucrose (Ishiguro and Nakamura, 1994). SPF1 was also found in cucumber (Kim D J et al., 1997, Gene, 185: 265-269) and parsley (Rushton et al., 1996, EMBO J., 15: 5690-5700), which commonly encode a WRKY domain transcription factor.
The WRKY factor has an amino acid sequence of WRKYGQK, together with a zinc-finger-like motif at the N-terminal end. The WRKY domain has a high binding ability to a specific base sequence of (T) (T)TGAC(C/T) known as the W-box. The prior reported WRKY proteins all have one or two WRKY domains in the molecule and are classified, according to the number of the WRKY domains and the structure of zinc-finger-like motif, into three groups (Eulgem et al., 2000, Trends plant sci. 5: 199-206). To date, more than 70 WRKY genes were found in Arabidopsis thaliana, and these WRKY proteins function as transcription inducing or inhibitory factors. It was reported that the transcription of these genes is rapidly and strongly induced by wounds, pathogenic infection and non-biological stresses (Asai et al., 2002, Nature, 415: 977-983), and plays an important role in the defense mechanism of plants. Also, some WRKY genes are involved in embryogenesis, seed coat, trichome development and senescence (Miao et al., 2004, Plant Mol. Biol. 55(6): 853-867).